Solar systems come in two generic types, fixed flat-plate one-sun panels using cheaper less efficient solar cells (say 16% efficient at one sun) and tracking concentrator systems using expensive high efficiency cells (say 42% efficient at 500 suns). Fixed flat-plate systems have a lower initial cost and are often used in residential applications. Tracking systems have a much higher initial cost and are normally used in larger-scale utility-level installations. The key metric can be $/watt and therefore more costly cells may be justified in tracking system embodiments, in some cases.